OPTICAL PICKUP
An optical pickup having a first diffractive optical element that acts on light of a specific wavelength, the first diffractive optical element being arranged in an optical path for guiding reflected light from an optical disk to a photodetector. The photodetector comprises a first detector including a first main light receiver divided into a two-by-two square matrix, and a second detector including a second main light receiver divided into a two-by-two square matrix, the second detector being arranged in a row with the first detector. When the light of a specific wavelength is used, zero-order light of the first diffractive optical element is received by the first main light receiver, +first-order light (or −first-order light) diffracted by the first diffractive optical element is received by the second main light receiver, and a tracking error signal is generated using an output signal from the second main light receiver.
This application is based on Japanese Patent Application No. 2010-250921 filed on Nov. 9, 2010, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to an optical pickup used for reading information recorded on an optical disk and/or writing information to an optical disk.
2. Description of Related Art
There are known in the art optical pickups used for reading and writing information from and to Blu-Ray discs (“BDs”), digital versatile discs (“DVDs”), compact discs (“CDs”), and other optical disks. The optical pickup includes an objective lens for focusing light emitted from a light source on an information-recording surface of the optical disk.
The position of the light spot focused by the objective lens must be controlled in the following manner in the case that the optical pickup is used to read information from the optical disk or write information to the optical disk. First, the light spot focused by the objective lens must be controlled so as to be constantly present on the information-recording surface of the optical disk (focusing control). Second, the light spot focused by the objective lens on the information-recording surface must be controlled so as to constantly follow the tracks formed on the optical disk (tracking control).
Therefore, the optical pickup is provided with a photodetector (PDIC) for receiving reflected light (returned light) that has been reflected by the optical disk. In the optical pickup, a focus error (FE) signal for expressing the degree of offset between the light spot and the information-recording surface (target information-recording surface), and a tracking error (TE) signal for expressing the degree of offset between the light spot and the track (target track) are calculated based on signals outputted by the photodetector. A control (focusing control) is performed for moving the position of the objective lens in the focus direction on the basis of the FE signal, and a control (tracking control) is performed for moving the position of the objective lens in the tracking direction on the basis of the TE signal.
The focus direction is a direction perpendicular to the information-recording surface of the optical disk, and the tracking direction is a direction parallel to the radial direction (Rad direction) of the optical disk.
Known methods for calculating the TE signal have conventionally included the push-pull (PP) technique and the differential push-pull (DPP) technique. There is also a method for generating a TE signal using a holographic element (diffractive optical element) arranged in the optical path over which the returned light passes, as described in, for example, Japanese Patent Application No. 2010-44847.
SUMMARY OF THE INVENTIONThe DPP technique is a scheme for minimizing the effect of lens shift (offset of the objective lens in the tracking direction) and obtaining a TE signal, and is widely used in the art in optical pickups. However, there are cases in which a TE signal cannot be generated using the DPP technique in, for example, an optical pickup that has two objective lenses wherein the two objective lenses are arranged in a row in the tangential direction (Tan direction). A related description is provided below with reference to
In the case that the two objective lenses 101, 102 are configured in a Rad arrangement (see
In the case that the two objective lenses 101, 102 are configured in a Tan arrangement, a configuration is typically assumed in which the center TO1 of one of the objective lenses (in this case, the first objective lens 101) is situated on a reference plane R that passes through the center of the turntable 104 (which is the same as the center O of the optical disk), as shown in
In this case, the relative positional relationship between the track TR and the light spot SP focused by the objective lens (first objective lens 101) which has its center on the reference plane R does not change even if the optical pickup 103 moves in the Rad direction, as shown in
In the case that the objective lens (second objective lens 102) having its center in a position offset from the reference plane R is used, the relative positional relationship between the track TR and the light spot SP focused by the objective lens varies depending on the position of the optical pickup 103 in the Rad direction (see
With the DPP technique, light emitted from a light source is divided into three beams composed of a main beam and two sub beams by a diffractive lattice (grating), and used to irradiate the optical disk. The sub beams generated by the grating must be accurately adjusted for position in the direction that cuts across the track TR (pitch direction). However, in the case that the two objective lenses 101, 102 are in a Tan arrangement, the relative positional relationship between the light spot SP and the track TR varies depending on the position of the optical pickup 103, as described above, in the case that the objective lens having its center in a position offset from the reference plane R (the second objective lens 102 in the example described above) is used. Therefore, when the two objective lenses 101, 102 are configured in a Tan arrangement, the conventional DPP technique cannot be used in the case that the objective lens having its center in a position offset from the reference plane R is used.
It is possible to consider applying a scheme in which a holographic element (diffractive optical element) is used as a scheme for obtaining the TE signal using a method other than the DPP technique; for example, as described in Japanese Patent Application No. 2010-44847. However, according to the configuration in Japanese Patent Application No. 2010-44847, it is necessary to prepare a special photodetector (PDIC) whose configuration is different from that of the light receiver used in the conventional DPP technique or the like (a configuration in which there are aligned in a row a main light receiver quadrisected into a two-by-two square matrix and two sub light receivers provided on either side of the main light receiver). In other words, a problem is presented in that a photodetector must be newly fabricated, costs increase, and a long manufacturing time period is required.
However, with the scheme in which a TE signal is obtained using a diffractive optical element arranged in the optical path over which light reflected from the optical disk is guided toward the photodetector, the light utilization ratio is improved and other advantages are presented. Therefore, it would be very useful to be able to use a conventionally used general-purpose photodetector while making use of a diffractive optical element.
In view of the above points, an object of the present invention is to provide an optical pickup that is provided with a diffractive optical element in the optical path through which returned light passes, and in which a tracking error signal can be suitably obtained using a photodetector having a general-purpose light-receiving pattern.
In order to achieve the above-stated object, the present invention provides an optical pickup having a first diffractive optical element that acts on light of a specific wavelength, the first diffractive optical element being arranged in an optical path for guiding reflected light from an optical disk to a photodetector, wherein the photodetector comprises a first detector including a first main light receiver divided into a two-by-two square matrix, and a second detector including a second main light receiver divided into a two-by-two square matrix, the second detector being arranged in a row with the first detector; and in the case that the light of a specific wavelength is used, zero-order light of the first diffractive optical element is received by the first main light receiver, a first-order light among the ±first-order light diffracted by the first diffractive optical element is received by the second main light receiver, and a tracking error signal is generated using an output signal from the second main light receiver.
In accordance with the present configuration, a tracking error (TE) signal generated using a diffractive optical element can be acquired by a photodetector having a general-purpose light-receiving pattern in the case that light of a specific wavelength is used. Also, in the case that light of a specific wavelength is used, the light utilization ratio can be increased in comparison with the case in which a TE signal is generated using, for example, the DPP method, because the configuration is one in which a TE signal is obtained using a diffractive optical element arranged in the optical path through which returned light passes.
In the optical pickup of the configuration described above, the first main light receiver and two first sub light receivers arranged so as to be disposed on both sides of the first main light receiver may be aligned in a row in the first detector; the second main light receiver and two second sub light receivers arranged so to be disposed on both sides of the second main light receiver may be aligned in a row in the second detector; and the first main light receiver and the second main light receiver may be adjacently arranged. The present configuration corresponds to an example of the case in which a photodetector having a conventionally used general-purpose light-receiving pattern is used.
In the optical pickup of the above-described configuration, it is preferred that reflected light reflected by the optical disk include zero-order light and ±first-order light, and that the first diffractive optical element have a first diffraction region in which light is incident, the light resulting from superposition of zero-order light and +first-order light included in the reflected light; a second diffraction region in which light is incident, the light resulting from superposition of zero-order light and −first-order light included in the reflected light, and the second diffraction region being symmetrically arranged in relation to the first diffraction region; a third diffraction region wherein the ±first-order light included in the reflected light is entirely not or substantially not incident, and the zero-order light is incident, the third diffraction region being located on the same side as the first diffraction region relative to the line of symmetry between the symmetrically arranged first diffraction region and second diffraction region; and a fourth diffraction region wherein the ±first-order light included in the reflected light is entirely not or substantially not incident, and the zero-order light is incident, the fourth diffraction region being located on the same side as the second diffraction region relative to the line of symmetry; wherein either +first-order light or −first-order light from among the ±first-order light diffracted by the diffraction regions of the first diffractive optical element arrives in separate regions of the second main light receiver. In accordance with the present configuration, it is possible to obtain a suitable TE signal in which the effects of lens shift, optical disk tilt, and the like are inhibited.
The optical pickup of the above-described configuration preferably further comprises a lens arranged between the first diffractive optical element and the photodetector and provided with a cylindrical surface, wherein the diffraction regions of the first diffractive optical element are provided so that an astigmatism of reverse polarity relative to an astigmatism imparted by the cylindrical surface is imparted to first-order light that is diffracted by the diffraction regions and arrives at the second main light receiver. With such a configuration, first-order light (diffracted light) that arrives at the second main light receiver from the first diffractive optical element can be focused without departure from the target region.
The optical pickup of the above-described configuration may further comprise a first light source for emitting light of a first wavelength; a second light source for emitting light of a second wavelength; and a third light source for emitting light of a third wavelength, wherein a tracking error signal is generated using a signal outputted from the first detector in the case that the light of a first wavelength is used; a tracking error signal is generated using a signal outputted from the second detector in the case that the light of a second wavelength is used; and the light of a third wavelength is the light of a specific wavelength. In such a configuration, for example, the first light source can be a DVD-compatible light source, the second light source can be a CD-compatible light source, and the third light source can be a BD-compatible light source.
In the optical pickup of the above-described configuration, light of a first wavelength emitted from the first light source may be divided into a main beam and two sub beams by a second diffractive optical element and be incident on the optical disk; and a tracking error signal may be obtained by the differential push-pull (DPP) method in the case that the light of a first wavelength is used. In the present configuration, the effect of the first diffractive optical element is inhibited in the case of DVD compatibility, and a TE signal produced by the DPP method can be suitably obtained.
The optical pickup of the above-described configuration may further comprise: a first objective lens for focusing the light of a first wavelength and the light of a second wavelength on an information-recording surface of the optical disk; and a second objective lens for focusing the light of a third wavelength on an information-recording surface of the optical disk. In the case of this configuration, the first objective lens and the second objective lens may be arranged in a row in a direction that is parallel to a tangential direction of tracks on the optical disk. The present configuration is one in which the two objective lenses are in a so-called Tan arrangement, and the present invention is advantageous for an optical pickup having such a configuration.
In accordance with the present invention, it is possible to provide an optical pickup that is provided with a diffractive optical element in the optical path through which returned light passes, and in which a tracking error signal can be suitably obtained using a photodetector having a general-purpose light-receiving pattern. The present invention is applicable to optical pickups in which two object lenses are in a Tan arrangement.
Embodiments of the optical pickup of the present invention are described in detail below with reference to the drawings. The optical pickup of the present embodiment is compatible with three types of optical disk: BD, DVD, and CD.
The optical pickup 1 of the present embodiment has a configuration provided with a pickup base 10, and an objective lens actuator 30 fixedly arranged on the pickup base 10, as shown in
Bearing units 10a, 10b are provided to the left and right end parts of the pickup base 10. The pickup base 10 is slidably supported on guide shafts 100 (indicated by broken lines in
The objective lens actuator 30 is a device that allows the two objective lenses 18, 21 provided to the optical system of the optical pickup 1 to move in the focus direction and the tracking direction (same as the Rad direction).
In the optical pickup 1, focusing control must be carried out so that the focal position of the objective lenses 18, 21 conforms to the information-recording surface RS of the optical disk D when information is to be read or written. Also, in the optical pickup 1, tracking control must be carried out so that the position of the light spot focused on the information-recording surface RS of the optical disk D by the objective lenses 18, 21 follows the tracks on the optical disk when information is to be read or written. The objective lens actuator 30 allows these focusing and tracking controls to be carried out.
The objective lens actuator 30 of the present embodiment is also capable of demonstrating a tilt function for adjusting the slope in relation to the optical axis of the objective lenses 18, 21. The objective lens actuator 30 has a lens holder for holding the objective lenses 18, 21 and is configured to swingably support the lens holder using a wire. Such a type of objective lens actuator is well known, and a detailed description is therefore omitted herein.
The laser light emitted from the first semiconductor laser 11 passes through a grating 12 (an example of the second diffractive optical element of the present invention) and arrives at a beam splitter 13. The grating 12 is arranged to divide the DVD laser light into three beams composed of a main beam and two sub beams. The laser light reflected by the beam splitter 13 passes through a polarized beam splitter 14, a ¼ wavelength plate 15, and a collimating lens 16, is reflected by a first raising mirror 17, and arrives at the first objective lens 18 above the first raising mirror 17. The first objective lens 18 has a function for focusing incident laser light on the information-recording surface RS of the optical disk D (see
The laser light focused on the information-recording surface RS by the first objective lens 18 is reflected by the information-recording surface RS. The reflected light (returned light) passes through the first objective lens 18, is thereafter reflected by the first raising mirror 17, and is sequentially transmitted by the collimating lens 16, the ¼ wavelength plate 15, the polarized beam splitter 14, and the beam splitter 13. The laser light transmitted by the beam splitter 13 is focused on a photodetector 24 via a sensor optical system composed of a diffractive optical element 22 (an example of the first diffractive optical element of the present invention) and a sensor lens 23 that includes a cylindrical surface. The photodetector 24 functions as photoelectric conversion means for converting the received optical signal into an electric signal. The electric signal outputted from the photodetector 24 is sent to a signal processor 25. A replay signal, an FE signal, a TE signal, and the like are generated in the signal processor 25.
The controller (not shown) of the optical pickup 1 controls the driving of the objective lens actuator 30 on the basis of the FE signal and TE signal generated by the signal processor 25 in order to carry out focusing control and tracking control.
A second semiconductor laser 19 is capable of emitting BD laser light (for example, laser light in the 405-nm wavelength band; an example of light of a third wavelength of the present invention). The second semiconductor laser 19 is a light source for emitting light of a single wavelength, and is an example of the third light source of the present invention. The laser light emitted from the second semiconductor laser 19 is reflected by the polarized beam splitter 14, transmitted by the ¼ wavelength plate 15, the collimating lens 16, and the first raising mirror 17, and reflected by a second raising mirror 20. The laser light reflected by the second raising mirror 20 arrives at a second objective lens 21 above the second raising mirror 20. The second objective lens 21 has a function for focusing incident laser light on the information-recording surface RS of the optical disk D (see
The laser light focused on the information-recording surface RS by the second objective lens 21 is reflected by the information-recording surface RS. The reflected light (returned light) passes through the second objective lens 21, is then reflected by the second raising mirror 20, passes through the first raising mirror 17, and thereafter arrives at the photodetector 24 in the same manner as the case in which the laser light emitted from the first semiconductor laser 11 is reflected by the optical disk D. Also, the electric signal obtained by photoelectric conversion in the photodetector 24 is outputted to the signal processor 25.
The polarized beam splitter 14 is an optical member that acts on the BD laser light. Also, the collimating lens 16 is capable of moving in the optical axis direction M (left/right direction in
In the optical pickup 1 of the present embodiment, the optical axis of the BD laser light is provided so as to substantially match the optical axis of the DVD laser light. However, the BD laser light is adapted to be detected both by the first detector 241 and by the second detector 242, as described below.
The first detector 241 has a first main light receiver 241a and two first sub light receivers 241b, 241c, each of which is quadrisected into a two-by-two square matrix. The three light receivers 241a to 241c are aligned in a row, with the first main light receiver 241a in the middle. Also, the second detector 242 has a second main light receiver 242a quadrisected into a two-by-two square matrix, and two second sub light receivers 242b, 242c bisected into a bisected square. The three light receivers 242a to 242c are aligned in a row, with the second main light receiver 242a in the middle, so as to be parallel to the lineup of the first detector 241. The second detector 242 is arranged in a row with the first detector 241, and the second main light receiver 242a and the first main light receiver 241a are adjacent to each other. The first detector 241 and second detector 242 have a general-purpose light-receiving pattern conventionally used in the DPP method and the like.
In
In the optical pickup 1 having such a configuration, an FE signal produced by the so-called astigmatic method can be obtained using the computational formula shown in formula (1) in the case of DVD compatibility. A TE signal produced by the DPP method can be obtained using the computational formula shown in formula (2). These signals may be used to carry out focusing control and tracking control in which the objective lens actuator 30 is used.
FE=(A+C)−(B+D) (1)
TE=(A+D)−(B+C)−k((E+H+I+L)−(F+G+J+K)) (2)
The alphabetic letters used in formulas (1) and (2) indicate signals outputted from the regions of the light receivers 241a to 241c (light receivers of the first detector 241) shown in
In the optical pickup 1, an FE signal based on the so-called astigmatic method can be obtained using the computational formula shown in formula (3) in the case of CD compatibility.
FE=(a+c)−(b+d) (3)
The alphabetic letters used in formula (3) indicate signals outputted from the regions of the second main light receiver 242a (light receiver of the second detector 242) shown in
The TE signal in the case of CD compatibility can be obtained by the known differential phase detection (DPD) method by using a signal obtained from the second main light receiver 242a. However, the TE signal in the case of CD compatibility may also be obtained using the so-called three-beam method. The resulting FE and TE signals may be used to carry out focusing control and tracking control in which the objective lens actuator 30 is used.
In the optical pickup 1 of the present embodiment, the two objective lenses 18, 21 are in a Tan arrangement, and the center of the second objective lens 21 used in the case of BD compatibility is in a position offset from the reference plane R described above (see
In the case of BD compatibility as well, the FE signal can be obtained using the astigmatic method in the same manner as the case of DVD compatibility.
The tracks formed on the optical disk D act as a diffraction grating for the light incident on the optical disk D. Therefore, the returned light (reflected light) reflected by the information-recording surface RS of the optical disk D is divided, and zero-order light and ±first-order light are included in the returned light. The zero-order light and the ±first-order light are partially superimposed, and light resulting from superposition of the zero-order light and the ±first-order light is incident on the diffractive optical element 22, as shown in
The portion (present on the two end sides in the Rad direction) in which the zero-order light and the ±first-order light are superimposed corresponds to a region that includes an AC signal (track-crossing signal) component generated when the light incident on the optical disk D crosses a track. The portion in which the zero-order light and the ±first-order light are not superimposed is a region that indicates a luminous energy that corresponds to the reflectivity of the optical disk D. In other words, this portion does not include a track-crossing signal component, and corresponds to a region that includes an offset component produced, for example, by lens shift, the tilt of the optical disk D, or the like.
The diffractive optical element 22 that acts on light having a BD wavelength has a plurality of diffraction regions R1, R2, R3, R4 that each exhibit a different effect, as shown in
The substantially rectangular first diffraction region R1 is provided so that light composed of superimposed zero-order light and +first-order light diffracted by the tracks on the optical disk D can be incident on the region. The substantially rectangular second diffraction region R2 is provided so that light composed of zero-order light and −first-order light diffracted by the tracks on the optical disk D can be incident on the region. A rectangular region N that does not have a diffractive effect is present between the first diffraction region R1 and second diffraction region R2, which are aligned in the Rad direction. The first diffraction region R1 and second diffraction region R2 are arranged in line symmetry with respect to the line of symmetry CX, and have substantially the same area.
The third diffraction region R3, which is formed of two substantially rectangular regions, is provided so that the ±first-order light diffracted by the optical disk D is substantially not incident and zero-order light is incident. Also, the two regions constituting the third diffraction region R3 are adjacently arranged so as to be disposed on both sides of the first diffraction region R1 in relation to the direction perpendicular to the Rad direction. The fourth diffraction region R4, which is formed of two substantially rectangular regions, is provided so that the ±first-order light diffracted by the optical disk D is substantially not incident and zero-order light is incident. Also, the two regions constituting the fourth diffraction region R4 are adjacently arranged so as to be disposed on both sides of the second diffraction region R2 in relation to the direction perpendicular to the Rad direction. The third diffraction region R3 and the fourth diffraction region R4 are symmetrically arranged with respect to the line of symmetry CX in a mutually adjacent state, and the areas of the two are substantially the same.
The ±first-order light diffracted by the optical disk D is preferably not incident at all on the third diffraction region R3 and fourth diffraction region R4.
Next, the function of the diffractive optical element 22 provided to the optical pickup 1 will be described with reference to
As part of the returned light of the BD laser light, the zero-order light (the light transmitted by the diffractive optical element 22) in the diffractive optical element 22 forms a light spot on the first main light receiver 241a of the first detector 241, as shown in
As part of the returned light of the BD laser light, the +first-order light diffracted by the diffractive optical element 22 forms a light spot on the second main light receiver 242a of the second detector 242. More specifically, the +first-order light diffracted by the first diffraction region R1 is focused in region “a” of the second main light receiver 242a, as shown in
In other words, regions “a” and “b” are regions for receiving light that includes a track-crossing signal component (push-pull component). Also, regions “c” and “d” substantially do not include a push-pull component, and are regions for receiving light that mainly includes an offset component.
The diffraction regions R1 to R4 are provided so that an astigmatism of reverse polarity relative to the astigmatism imparted by the cylindrical surface of the sensor lens 23 is imparted to the +first-order light obtained by diffraction in the diffractive optical element 22. Therefore, the beam of +first-order light diffracted by the diffraction regions R1 to R4 of the diffractive optical element 22 is narrowed and focused in the regions “a” to “d” of the second main light receiver 242a.
The +first-order light diffracted by the diffraction regions R1 to R4 can thereby be prevented from departing from the regions “a” to “d” of the second main light receiver 242a, allowing light-receiving balance not to be offset. Also, the position of the photodetector 24 may, for example, become slightly offset due to temperature variation, vibration/impact, or the like, but adopting a configuration such as that of the present embodiment reduces the possibility that an offset will be produced in the light-receiving balance by such a positional offset.
As part of the returned light of the BD laser light, the −first-order light diffracted by the diffractive optical element 22 forms a light spot on the side opposite, as seen relative to the first main light receiver 241a, from the position in which the +first-order light forms a light spot. This −first-order light is stray light, is not received by either of the light receivers on the photodetector 24, and is not used in signal generation.
As described above, the diffraction regions R1 to R4 of the diffractive optical element 22 are provided so that an astigmatism of reverse polarity relative to the astigmatism imparted by the cylindrical surface of the sensor lens 23 is imparted to the +first-order light. Therefore, the −first-order light conjugate with the +first-order light forms a large light spot in comparison with the +first-order light. The optical pickup 1 is formed so that this enlarged light spot is not received by any of the light receivers.
In the description above, the +first-order light is received by the second main light receiver 242a and the −first-order light is stray light, but which type of first-order light is plus and which is minus varies depending on the criterion adopted. Therefore, the above arrangement can be redefined so that the −first-order light is received by the second main light receiver 242a and the +first-order light is stray light. In the case that the diffraction pattern formed in the diffraction regions R1 to R4 is configured to have a blazed grating, and the luminous energy of one type of first-order light from among the ±first-order light is set to be substantially zero, the other type of first-order lights can be received by the second main light receiver 242a.
With a configuration such as that described above, a TE signal in which lens shift, a tilt of the optical disk D, and other offset effects are inhibited can be obtained using formula (4) noted below in the case of BD compatibility.
TE=(a−b)−k′(d−c) (4)
The alphabetic letters used in formula (4) indicate, for example, signals outputted from the regions of the second main light receiver 242a (light receiver of the second detector 242) shown in
The coefficient k′ in formula (4) can be experimentally set to a suitable value that allows, for example, a lens shift or other offset to be produced and the offset to be canceled.
Also, with the optical pickup 1 of the present embodiment, the first detector 241 can be jointly used both for DVD compatibility and for BD compatibility, and the second detector 242 can be jointly used both for CD compatibility and for BD compatibility. Therefore, a switch (not shown) is suitably provided so that the signal is outputted in accordance with the type of optical disk.
In accordance with the optical pickup 1 of the present embodiment, a TE signal can be suitably obtained and suitable tracking control can be carried out in cases in which either of the objective lenses is used, even when the first objective lens 18 and the second objective lens 21 are in a Tan arrangement. This approach is extremely advantageous in terms of cost because a general-purpose light-receiving pattern (photodetector) conventionally used in the DPP method, three-beam method, or the like can be used even in a configuration in which a TE signal is obtained using the diffractive optical element 22 arranged in a position through which only returned light passes. In the case of BD compatibility, excellent results are obtained in terms of the light utilization ratio because the laser light used to irradiate the optical disk D is a single beam (a configuration in which light is not divided into three beams by a diffractive optical element and is not used to irradiate the optical disk).
Also, in the optical pickup 1 of the present embodiment, the diffractive optical element 22 is arranged in the optical path through which the returned light passes, producing the advantage that the effect of the diffractive optical element 22 can be inhibited to produce a TE signal in the case of DVD compatibility. This will be described with reference to
In the case of DVD compatibility, the DVD laser light emitted from the first semiconductor laser 11 is divided by the grating 12 into three beams composed of a main beam and two sub beams, and then used to irradiate the optical disk D. After being reflected by the optical disk D, the main beam is received by the first main light receiver 241a, one of the two sub beams (+first-order light) is received by the first main light receiver 241b, and the other (−first-order light) is received by the first main light receiver 241c.
The diffractive optical element 22 is configured so as to act on BD laser light. In this case, the diffraction ratio of the DVD laser light can be reduced in the diffractive optical element 22, but it is difficult to bring the diffraction ratio completely to zero. Therefore, light (stray light) diffracted by the diffractive optical element 22 is generated even when DVD laser light is used. However, the diffracted light (stray light) is not received by the first detector 241 in the same manner as in the case described for BD laser light (however, the diffraction angle varies due to the difference in wavelength), as shown in
The embodiment described above is an example of the present invention, and the optical pickup of the present invention is not limited to the configuration described above.
For example, the manner in which the +first-order light diffracted by the diffraction regions R1 to R4 is focused in either position (region) of the second main light receiver 242a is not limited to the configuration of the present embodiment and may be suitably modified. This is because the configuration for computing the signal to be outputted from the regions “a” to “d” of the second main light receiver 242a can be adapted by adopting a modification that differs from the case of the present embodiment.
The configuration (pattern) of the diffraction regions of the diffractive optical element 22 described above is merely an example, and the configuration of the diffraction regions can be suitably modified. For example, region N (see
Also, for example, region N devoid of diffraction grooves in the center part of the diffractive optical element 22 of the present embodiment may be bisected (divided along the line of symmetry CX), as shown in
In the embodiment described above, the TE signal is obtained by the DPP method in the case of DVD compatibility; and the TE signal is obtained using diffracted light from the diffractive optical element 22 in the case of BD compatibility. However, an opposite configuration may be used (the DPP method is used for BD compatibility, and the diffractive optical element 22 is used for DVD compatibility). In this case, for example, the grating 12 of the present embodiment is modified so as to be arranged in front of the second semiconductor laser 19 that emits BD laser light.
The embodiment described above has a configuration in which the first objective lens 18 and the second objective lens 21 are in a Tan arrangement, but the applicable range of the present invention is not limited to this configuration. In other words, the present invention can also be applied to, for example, the case in which the two objective lenses 18, 21 are in a Rad arrangement. The present invention may also be applied to a case in which the number of objective lens provided to the optical pickup is other than two.
It is also apparent that the types of optical disks that can be used with the optical pickup to which the present invention is applied are not limited to those in the embodiment described above.
The present invention is advantageous for an optical pickup in which BD, DVD, and CD can be interchanged.
Claims
1. An optical pickup comprising:
- a photodetector for receiving reflected light from an optical disk; and
- a first diffractive optical element that acts on light of a specific wavelength, the first diffractive optical element being arranged in an optical path for guiding reflected light from the optical disk to the photodetector, wherein
- the photodetector has:
- a first detector including a first main light receiver divided into a two-by-two square matrix, and
- a second detector including a second main light receiver divided into a two-by-two square matrix, the second detector being arranged in a row with the first detector; and
- in the case that the light of a specific wavelength is used,
- zero-order light of the first diffractive optical element is received by the first main light receiver,
- a first-order light among the ±first-order light diffracted by the first diffractive optical element is received by the second main light receiver, and
- a tracking error signal is generated using an output signal from the second main light receiver.
2. The optical pickup of claim 1, wherein
- the first main light receiver and two first sub light receivers arranged so as to be disposed on both sides of the first main light receiver are aligned in a row in the first detector;
- the second main light receiver and two second sub light receivers arranged so to be disposed on both sides of the second main light receiver are aligned in a row in the second detector; and
- the first main light receiver and the second main light receiver are adjacently arranged.
3. The optical pickup of claim 1, wherein
- reflected light reflected by the optical disk includes zero-order light and ±first-order light;
- the first diffractive optical element has:
- a first diffraction region in which light is incident, the light resulting from superposition of zero-order light and +first-order light included in the reflected light,
- a second diffraction region in which light is incident, the light resulting from superposition of zero-order light and −first-order light included in the reflected light, and the second diffraction region being symmetrically arranged in relation to the first diffraction region,
- a third diffraction region wherein the ±first-order light included in the reflected light is entirely not or substantially not incident, and the zero-order light is incident, the third diffraction region being located on the same side as the first diffraction region relative to the line of symmetry between the symmetrically arranged first diffraction region and second diffraction region; and
- a fourth diffraction region wherein the ±first-order light included in the reflected light is entirely not or substantially not incident, and the zero-order light is incident, the fourth diffraction region being located on the same side as the second diffraction region relative to the line of symmetry; wherein
- either +first-order light or −first-order light from among the ±first-order light diffracted by the diffraction regions of the first diffractive optical element arrives in separate regions of the second main light receiver.
4. The optical pickup of claim 3, further comprising a lens arranged between the first diffractive optical element and the photodetector and provided with a cylindrical surface, wherein
- the diffraction regions of the first diffractive optical element are provided so that an astigmatism of reverse polarity relative to an astigmatism imparted by the cylindrical surface is imparted to first-order light that is diffracted by the diffraction regions and arrives at the second main light receiver.
5. The optical pickup of claim 1, further comprising:
- a first light source for emitting light of a first wavelength;
- a second light source for emitting light of a second wavelength; and
- a third light source for emitting light of a third wavelength, wherein
- a tracking error signal is generated using a signal outputted from the first detector in the case that the light of a first wavelength is used;
- a tracking error signal is generated using a signal outputted from the second detector in the case that the light of a second wavelength is used; and
- the light of a third wavelength is the light of a specific wavelength.
6. The optical pickup of claim 5, wherein
- light of a first wavelength emitted from the first light source is divided into a main beam and two sub beams by a second diffractive optical element and is incident on the optical disk; and
- a tracking error signal is obtained by the differential push-pull (DPP) method in the case that the light of a first wavelength is used.
7. The optical pickup of claim 5, further comprising:
- a first objective lens for focusing the light of a first wavelength and the light of a second wavelength on an information-recording surface of the optical disk; and
- a second objective lens for focusing the light of a third wavelength on an information-recording surface of the optical disk.
8. The optical pickup of claim 7, wherein the first objective lens and the second objective lens are arranged in a row in a direction that is parallel to a tangential direction of tracks on the optical disk.
Type: Application
Filed: Nov 8, 2011
Publication Date: May 10, 2012
Inventors: Atsushi IKEDA (Osaka), Kenji Okamoto (Osaka)
Application Number: 13/291,243
International Classification: G11B 27/36 (20060101); G11B 7/1374 (20120101); G11B 7/1353 (20120101);